Multiple exposure method and apparatus

ABSTRACT

A photoelectrophoretic ink is subjected to light and electric field a plurality of times during a single scanning pass over the ink by a plurality of web electrodes positioned within the boundaries of a narrow slit scanning light image.

Emited States Patent E naczak Jan. 14, 1975 [5 MULTIPLE EXPOSURE METHODAND APPARATUS [56] References Cited [75] Inventor: Raymond K. Egnaczak,Williamson, UNITED STATES PATENTS N.Y. 3,703,335 11/1972 Hoffman 355/4[73] Assignee: )gig): Corporation, Stamford, Primary Examiner john M.Hora Attorney, Agent, or FirmJames J. Ralabate; David C. [22] Filed:NOV- 21, 9 Petre; Charles E. Smith [21] Appl. No.: 309,044 ABSTRACTRelated Application Data A photoelectrophoretic ink is subjected tolight and [62] Dlvlslo" of 104333 1971' PaL electric field a pluralityof times during a single scan- 317l9*484 ning pass over the ink by aplurality of web electrodes [52] U S Cl 355/4 positioned within theboundaries of a narrow slit scanc l 51 1111.01 603g 13/10 mags [58}Field of Search 355/4; 96/1, 1.2, 1.3 7 Claims, 4 Drawmg FiguresPATENTEI] JAN I 41975 SHEEI 1 OF 2 PATENTED 1 4975 3,860 337 sum 2 or 2I FIG. 4

MULTIPLE EXPOSURE METHOD AND APPARATUS I This is a division ofapplication Ser. No. 104,333, filed Jan. 6, 1971, now U.S. Pat. No.3,719,484.

BACKGROUND OF THE INVENTION This invention relates to imaging systemsand, in particular, to photoelectrophoretic imaging systems andapparatus.

Images are formed in the photoelectrophoretic process by exposing aphotoelectrophoretic ink to electromagnetic radiation (hereafterreferred to as light) and electric field. The ink is made ofelectrically photosensitive particles bearing a net electrical chargesuspended in an electrically insulating liquid. Inks having differentcolor particles are capable of producing full color images withconventional full color inks using cyan, magenta and yellow particles.An extensive description of the photoelectrophoretic process, materialsand apparatus is given in U.S. Pat. Nos. 3,384,488; 3,384,565;3,384,566; and 3,393,933, the disclosures of which are incorporatedherein by reference. Briefly, the photosensitive particles are attractedto a transparent electrode by an electric field established between itand another electrode forming a dense layer of ink particles. This layerof particles is exposed to a pattern of liqht causing the light struckphotosensitive particles to change charge polarity driving them from thetransparent electrode. The ink particles remaining on the transparentelectrode comprise a positive image of the light pattern and the exposedink particles driven away comprise a negative image. The image and colorquality of ink images produced from a polychromatic ink are improved byrepeating the exposure and the application of electric field. In simplesystem configurations using flat plate transparent electrodes traversedby roller electrodes, the multiple exposure and application of field isnot a particularly difficult requirement to meet. However, in moresophisticated systems wherein full frame light patterns are not used,the methods and apparatus for re-exposing an ink image become relativelycomplex.

Accordingly, it is an object of the present invention to simplify theoptics required for performing multiple exposures of aphotoelectrophoretic ink.

More broadly, it is an object of this invention to improve thephotoelectrophoretic imaging process.

Another object of the invention is to devise novel methods and apparatusfor subjecting photoelectrophoretic ink and ink images to electricfields.

A specific object of the present invention is to devise methods andapparatus for performing multiple applications of light and field duringa single scanning pass.

Another object of the invention is to devise method and apparatus forinhibiting corona currents between closely spaced electrodes.

These and other objects of the instant invention are accomplished bypositioning a plurality of web electrodes into a very narrow exposureregion. The electrodes and exposure region are moved relative to atransparent electrode to form a full frame image on the transparentelectrode in a line by line fashion during a single scanning pass.Exposure is made by projecting a light pattern through a narrow slit andthrough the transparent electrode to the photoelectrophoretic ink. Theweb electrodes include small rollers or squeegees positioned within thewidth of the exposure region as defined by the slit. Each roller has aweb passed around it to carry away exposed particles driven toward itfrom the transparent electrode. A liquid is usually applied to each webthat is the same or similar to the electrically insulating liquidcarrier in the ink. The dielectric properties of the liquid suppresscorona currents in the vicinity outside the nips formed between the weband transparent electrodes.

DESCRIPTION OF THE DRAWINGS Other objects and features of the presentinvention will be apparent from a further reading of the description andfrom drawings which are:

FIG. 1 is a schematic, side elevation view of a photoelectrophoreticimaging system having a transparent drum electrode and continuous webelectrodes according to the present invention.

FIG. 2 is a schematic, partial side elevation view of the exposuresection of FIG. 1 having a modified web electrode structure.

FIG. 3 is a schematic, partial side elevation view of aphotoelectrophoretic imaging system having a transparent flat plateelectrode and consumable web and squeege electrodes according to thepresent invention.

FIG. 4 is a schematic, enlarged, partial view of a squeege electrodeshown in FIG. 3.

DESCRIPTION OF THE EMBODIMENTS Polychromatic imaging systems of thepresent type normally include slit scan exposure optics for projecting alight pattern to the nip, i.e., interface, between two electrodes. Theelectrodes have large potential differences applied between them toestablish a field sufficiently high for imaging. The slit scan optics iseven used in systems having flat electrode shapes if images are madefrom opaque originals. The slit scan optics are used in the lattersystem because the light intensity required to flood a narrow strip isfar less than that needed for full frame image projection. Furthermorebecause image quality and color are improved with multiple exposures tolight and field, the color system often includes means for projectingtwo or more slit scan images to two or more nips. An alternative to thesimultaneous projection of multiple slit images is to recycle thescanning apparatus to make several scanning passes over an ink image.The present photoelectrophoretic system solves these and other problemsby accomplishing a multiple exposure to light and field with a singlescanning pass of a slit image.

FIG. 1 illustrates the web electrodes of the present invention and howseveral of them are positioned within the scanning region to allowmultiple imaging, i.e., multiple exposure to light and field, during asingle scanning pass. The relative size of some of the'elements areexaggerated to clarify their function. The drum 1 is a transparentelectrode ori which color ink images are formed from thephotoelectrophoretic ink 2 contained in trough 3. The ink is applied tofirst imaging electrode 4 which carries the ink to the nip between itand the drum for the first exposure to light and field. A second andthird exposure to light and field is made as the ink image on the drummoves past the second 5 and third 6 imaging electrodes. The troughts 8and 9 contain electrically insulating liquids 10 and 11 (normally thesame as the insulating liquid of ink 2) which are carried to the nipsbetween electrodes 5 and 6 and the drum to assist the migration of inkparticles from the drum to an imaging electrode. The liquids and 11 alsoact to suppress corona currents in regions of high field outside thenips.

A narrow image pattern is projected to the ink 2 by appropriate opticalapparatus through the aperture or slit 13 formed between light stops 14and 15. The width of the slit 13 substantially defines the imagingregion which is small compared to the circumference of the drum 1. Forexample, when the circumference of drum 1 is in the order of 24-30inches, the width of the aperture is in the order of /2 to 2 inches,meaning that the exposed arc of the drum is relatively flat and anyoptical distortion due to curvature is negligible. Also, the width ofthe aperture is adequate for positioning at least two imaging electrodesopposite to it.

The ink image formed on the drum during a single rotation or scanningpass is transferred to a web 16 between the transfer electrode 17 anddrum 1. The transfer electrode has a potential coupled to it toestablish an electric field polarity to pull the ink particlescomprising the image toward the web. The transferred image may bepermanently fixed to the web by appropriate means.

The drum 1 is a complete 360 or partial transparent glass cylinderhaving a transparent conductive material (e.g., tin oxide) on itsoutside periphery. Conventionally, the conductive layer on the drum iselectrically grounded with potentials substantially above and belowground being coupled to the imaging, transfer and other (e.g., acleaning member) electrodes.

The imaging electrodes 4, 5 and 6 are substantially identical, therebyrequiring a detailed description of only one of them. The first imagingelectrode 4 includes a thin conductive web 18 formed into a continuousloop supported between rollers 19 and 20. Roller 20 is coupled to anappropriate mechanical energy source in order to rotate the websynchronously with the drum so as to establish a near zero relativevelocity between the web and drum. Roller 20 is also coupled to anappropriate electrical energy source that maintains the web at a highpotential. Web 18 has an insulating outer surface facing drum 1 toprevent shorting the potentials coupled to the drum and web. Thethickness of the insulating material is preferably small for high fieldin the nip between the web and drum and for high dielectric strength.The metal web may be comparatively larger for yielding mechanicalstrength to the web. Generally, the total web thickness is in the orderof 1/32 to 1/64 inch and, consequently, is negligible compared to thewidth of aperture 13.

A roller 19 is a small diameter roller journaled for rotation at the tipof a support block that has a cylindrical recess for supporting theroller. The small roller is not coupled to a power source but is rotatedby the movement of a web 18. The diameter of a small roller 19 isgenerally of the order of 5 8 to A inch for the 24-30 inch drummentioned earlier. With spacings of approximately Vs inch betweenelectrodes 4 and 5 and between electrodes 5 and 6, the three electrodes,considering web thickness, roller diameter and electrode spacing, occupya region approximately one inch wide.

The web configuration for the three imaging electrodes 4, 5 and 6 issignificant because the webs carry away migrated ink particles toconvenient locations for cleaning and wetting. The webs of the threeelectrodes converge to close spacing at the imaging region adjacentaperture 13 but they diverge to large spacings to permit cleaning andwetting at the troughs 3, 8 and 9. The brushes 26, 27 and 28 representappropriate cleaning apparatus for removing ink particles from thesurface of the electrodes. The cleaning function can be eliminated ifconsumable webs are used in place of the continuous webs. The wettingrollers 29, 30 and 31 represent appropriate apparatus for applying theliquids 10, 11 and 2, respectively, to the three electrodes 4, 5 and 6.

An alternative embodiment to that shown in FIG. 1 is to have inkingapparatus similar to wetting roller 31 and trough 3 positioned to applyink 2 directly to the drum 1 at a location counterclockwise from theaperture 13 but before the transfer roller 17. This configuration ishelpful for simplifying the apparatus for cleaning the surface ofelectrode 4 because more space is available when the inking apparatus isnot present.

Another alternative embodiment to that shown in FIG. 1 is to employcorotrons (electrostatic charging devices of the type described in US.Pat. No. 2,836,725) to establish the electric fields. The conductivewebs 18 on the electrode 4, 5 and 6 are coupled to ground as well as thedrum 1 and the corotrons are spaced from a web to deposit charge on theinsulating surface of the web before it enters the nip. The charge isattracted to the grounded web whether an ink 2 or liquid 10 or 11 is onan electrode and is deposited in quantities such that the field in thenip is adequate for imaging to take place. This method of establishingthe field is advantageous in that the thickness of the web can begreater for mechanical strength without fear of lowering field strengthin the nip for practical potentials.

FIG. 2 illustrates a modified web electrode structure for a system likethat shown in FIG. 1. Here only two electrodes are illustrated butmorecould be used. The idea in this embodiment is to expand the width ofeach individual nip between an imaging electrode and the drum. Imagingelectrodes 36 and 37 are composed of webs 38 and 39 similar to a web 18being supported by a drive roller like a roller 20 but by two small idlerollers (rollers 40-43) instead of a small roller 19.

FIG. 2 illustrates another modification other than the additional numberof small rollers 40-43. The other modification includes the meniscusforming dielectric wedge shaped members 44 and 45. The wedges areclosely spaced from webs 38 and 39 to cause ink or liquid to form ameniscus that prevents air breakdown. The potentials coupled to the websare sufficient to cause corona currents in regions between the webs anddrum where separated by air or other ambient gas. The wedges are highdielectric constant materials that enable the majority of the field tobe applied across it rather than the surrounding air. In addition, sincethe liquid fills the gap between the wedge and the web, air is excludedin regions outside the nip where the web and drum are closely spaced. Atfurther spacings between the web and drum the field strength isinsufficient to cause significant corona currents. The corona currentsare undesirable at the entrance to an imaging electrode nip because ittends to reduce'field strength, imparts undesirable polarities to theink particles and other reasons. The wedge 45 between webs 38 and 39 isused principally to insure the space between rollers 41 and 42 is filledwith liquid and/or ink. The webs are generally at the same potentials sothe field is very low or zero in the spaces between the web 38 and 39but the field is high in the space between rollers 41 and 42 close tothe drum 1.

The electrode configuration of FIG. 2 offers great versitility forvarying the angles 49-52 that the webs make with the drum, as comparedto a horizontal reference line. The large angle 52 of web 38 as itenters the nip enables wedge 44 to be removed because the web and drumapproach each other gradually. The air space at the entrance can befilled with a liquid for small spacings so that significant coronacurrents do not occur. On the other hand, angle 49 may be any desiredangle that leads to gradual or abrupt separation between web 39 and drum1 because the photoelectrophoretic process is not normally adverselyaffected by corona current generated at the exit to electrode nips. Theangles 50 and 51 are generally selected for convenience and for insuringthe presence of a liquid in the region close to drum 1 between rollers41 and 42.

The velocity at which the slit image, projected through aperture 13, ismoved past electrodes 4, 5 and 6 in FIG. 1 and electrodes 36 and 37 inFIG. 2, is selected to be substantially equal to the velocity of a pointon the periphery of the drum 1.

FIG. 3 illustrates a photoelectrophoretic system wherein a flattransparent conductive electrode 56 has a slit image projected throughit to the photoelectrophoretic ink 57. The aperture 58 formed betweenlight stops 59 and 60 defines the width of the slit image which ispreferably on the order of /2 to 2 inches. The squeege electrodeassembly 62 includes a conductive body 63 and two or more (here four)squeegee blades 6466 all spaced to fit within the imaging region definedby aperture 58. The insulating web 69 is woven around the squeegeeblades 64-66 and pulled over the blades at a velocity that issubstantially equal and opposite to the translational velocity of theassembly relative to the transparent electrode 56. The plate 56 isgrounded and a high potential is coupled to the conductive body ofassembly 62 giving rise to high electric fields in the nips between thetips ofthesqueege blades and the transparent electrode.

An insulating liquid similar to that in ink 57 is applied to the web 69before it enters the nips of the various blades to assist the migrationof ink particles, minimize air breakdown and otherwise assist themultiple exposure process for the production of high quality images.

FIG. 4 is an enlarged view of squeege blade 65 which is typical of theother blades. The web 69 is wrapped around idle rollers 70 to reduce themechanical drag on the web. The tip of each blade may include acompliant conductive member 70 such as conductive rubber to enable theweb to be pushed closely to the transparent electrode generallyuniformly along the length of the blade.

Whereas specific embodiments of the present invention have been given,it is apparent that modifications conventional to photoelectrophoreticimaging systems may be made without departing from the scope of theinvention. The principal concept of this invention is the use of webelectrodes in closely spaced regions to accomplish multiple exposures tolight and field during a single scanning pass.

What is claimed is:

1. In an imaging system of the type wherein full frame images are formedfrom polychromatic photoelectrophoretic ink by scanning said ink with anelectric field established in the nip between imaging and transparentelectrodes and with a narrow slit light image, the improvementcomprising a plurality of closely spaced imaging electrodes positionedwithin the boundaries of said slit light image wherein each imageelectrode includes a web forming a nip with the transparent electrodeand drive means for moving said webs relative to said transparentelectrode in a manner to establish a substantially zero relativevelocity between them in the nip during a scanning pass,

whereby said ink is exposed to a light image and subjected to electricfield a plurality of times during a single scanning pass.

2. The apparatus of claim 1 wherein said imaging electrode webs arewrapped around a small diameter roller to form the nip with thetransparent electrode.

3. The apparatus of claim 1 wherein said transparent electrode includesat least a section of a cylinder.

4. The apparatus of claim 1 wherein said transparent electrode includesa flat plate and said slit light image is formed with apparatusincluding means to flood with light a narrow region of a full frameopaque original.

5. The apparatus of claim 1 further including means for coating animaging electrode web with an electrically insulating liquid prior toentering said nip.

6. The apparatus of claim 1 wherein said imaging electrodes includecontinuous loop webs supported between small and large diameter rollerswith the small diameter roller, web thickness and electrode spacingbeing substantially equal to or less than the width of said slit lightimage.

7. The apparatus of claim 6 wherein said drive means includes means forcoupling a mechanical power source to said large roller for rotating theweb around the large and small rollers.

1. In an imaging system of the type wherein full frame images are formedfrom polychromatic photoelectrophoretic ink by scanning said ink with anelectric field established in the nip between imaging and transparentelectrodes and with a narrow slit light image, the improvementcomprising a plurality of closely spaced imaging electrodes positionedwithin the boundaries of said slit light image wherein each imageelectrode includes a web forming a nip with the transparent electrodeand drive means for moving said webs relative to said transparentelectrode in a manner to establish a substantially zero relativevelocity between them in the nip during a scanning pass, whereby saidink is exposed to a light image and subjected to electric field aplurality of times during a single scanning pass.
 2. The apparatus ofclaim 1 wherein said imaging electrode webs are wrapped around a smalldiameter roller to form the nip with the transparent electrode.
 3. Theapparatus of claim 1 wherein said transparent electrode includes atleast a section of a cylinder.
 4. The apparatus of claim 1 wherein saidtransparent electrode includes a flat plate and said slit light image isformed with apparatus including means to flood with light a narrowregion of a full frame opaque original.
 5. The apparatus of claim 1further including means for coating an imaging electrode web with anelectrically insulating liquid prior to entering said nip.
 6. Theapparatus of claim 1 wherein said imaging electrodes include continuousloop webs supported between small and large diameter rollers with thesmall diameter roller, web thickness and electrode spacing beingsubstantially equal to or less than the width of said slit light image.7. The apparatus of claim 6 wherein said drive means includes means forcoupling a mechanical power source to said large roller for rotating theweb around the large and small rollers.